The present invention relates to packaging materials of a type employing flexible, polymeric films. More specifically, the invention pertains to multilayer packaging films used in applications requiring a high degree of dimensional stability, i.e., both low shrinkage and low stretch, at elevated temperatures, and also a relatively low oxygen transmission rate.
Packaging applications requiring dimensionally stable films at high temperatures, e.g., up to about 150° C. and sometimes as high as about 180° C., include vertical form-fill-seal (VFFS) packaging for “hot fill” products, such as soups, sauces, jellies, beverages, and other liquified foods. As is well known, in VFFS packaging, a flowable product is introduced through a central, vertical fill tube and into a formed tubular film that has been heat-sealed transversely at its lower end and also longitudinally. Heat-sealing temperatures typically range from about 90 to about 150° C., and often reach or exceed 180° C. as packaging equipment operators attempt to make strong heat-seals, e.g., those that will seal through hot-fill product that inadvertently contaminates the seal area of the film.
After being filled, the package, in the form of a pouch, is completed by transversely heat-sealing the upper end of the tubular segment and simultaneously severing the pouch from the tubular film above it, usually by applying sufficient heat to melt through the tube above the newly formed upper heat-seal. If the film from which the package is made does not have sufficient dimensional stability, the package becomes distorted both from the heated product and from heat-sealing. Not only does package distortion ruin the aesthetic qualities of the package, e.g., by distorting any printed information or other labeling on the package, but it can cause the package to become misaligned in the packaging machinery, often resulting in ruined packages and costly downtime in production as misaligned packages become wedged between pieces of the machinery or when heat-sealing/severing equipment inadvertently contacts and melts through the walls of the package instead of sealing and severing at the periphery of the misaligned package as intended.
Similar considerations apply in VFFS and horizontal form/fill/seal (HFFS) packaging of flowable particulate products, e.g., shredded cheese, frozen chicken wings and nuggets, etc. Although such products are generally not filled while in a heated state, transverse and longitudinal heat-sealing and heat-severing alone are sufficient to cause package distortion, thereby making a film which is dimensionally stable at elevated temperatures highly desirable for such packaging applications.
Another packaging application for which high-temperature dimensional film stability would be desirable is when films are used as lidding materials for flexible packages, such as thermoformed pockets in which, e.g., hot dogs, lunch meats, etc., are contained; semi-rigid vacuum and/or gas-flushed packages for meat and poultry contained in a foam or other semi-rigid type tray; and rigid packages, e.g., for yogurt, custard and other dairy products contained in a rigid cup-like container. When lidding films are applied to such packages, heat is generally used to seal the film to the thermoformed container, tray, or cup in which the product is contained. As with the foregoing form-fill-seal packaging operations, high heat-seal temperatures are often employed in order to form strong heat-seals that will seal through particles or globs of food that contaminate the heat-seal area. Without sufficient dimensional stability, the lidding films can either stretch or shrink during the lidding process, resulting in incompletely sealed packages and distorted printed images on the films.
A further process necessitating dimensional stability at elevated temperatures is printing. Maintenance of color-to-color registration on the printing press is important, as is overall consistency of the “repeat length” of each printed image. Drying tunnel temperatures commonly reach temperatures of 120° F. or more. It is therefore preferred that the film have sufficient resistance to stretching, necking and other types of deformation at these temperatures so that registration is not lost, and that the repeat length of the images are consistently maintained on downstream packaging equipment, where it may again face elevated temperatures as noted above. When a film is printed, any deformation that occurs during the packaging process is particularly troublesome, in that the printed image becomes visibly distorted. Thus, maintaining dimensional stability under high-temperature packaging is especially important when the film is printed.
Another requirement of films used in many of the aforementioned packaging applications is a low transmission rate of oxygen in order to preserve and extend the shelf life of packaged food products. For many food products, the oxygen transmission rate (OTR) must be on the order of 40 cc/m2 per 24 hours at 1 atmosphere or less.
In order to achieve the above properties, many conventional packaging films used for such applications have been laminates, i.e., two or more film components that are adhesively bonded together, e.g., biaxially-oriented and heat-set polypropylene, polyester, or polyamide films that are adhesively laminated to a heat-sealable film where one of the laminated film components contains a low OTR material such as polyvinylidene chloride. However, adhesive lamination is expensive, due to the relatively high cost of the adhesives and the extra production steps required to produce the laminate, and the reliability of such adhesives is sometimes suspect, e.g., solvents from printing inks can reduce the bond-strength of the adhesives, leading to delamination. Further, certain types of adhesives contain low-molecular weight components that can migrate through films and contact the packaged food products.
Instead of using a laminate, it would be preferred to use a film that is fully coextruded, i.e., formed by extruding two or more polymeric materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a multilayer structure.
A proposed coextruded film having high-temperature dimensional stability and a low OTR includes a core layer of ethylene/vinyl alcohol copolymer (EVOH), a layer of amorphous polyamide to provide relatively high modulus at high temperatures, and a heat-sealing layer. While this film was able to address some of the problems detailed above, it was found that it did not provide a sufficiently wide temperature-processing window to make high quality packages from such film. That is, the glass transition temperature of the amorphous polyamide and the melting point of the heat-sealing layer were found to be too close to one another to provide a desired combination of good heat-sealability and dimensional stability at elevated temperatures.
Accordingly, there continues to be a need in the art for a coextruded, low-OTR multilayer film having a combination of high-temperature dimensional stability and low-temperature heat-sealability.